1. Field of the Invention
The present invention relates to an image processing technique for converting a gradation of an image.
2. Description of the Related Art
Conventionally, for the purpose of speeding up processing of images, a gradation of an image is converted into a low level to reduce processing loads in image processing. In converting an image into a low gradation, lower bits in each pixel of the image are eliminated. Japanese Patent Application Laid-Open No. 8-298665 discusses a method for detecting a motion vector in a moving image using such a technique.
However, in a case of image processing which detects feature quantity of an image such as motion vector detection, if the gradation reduction is performed by deleting lower bits, minute signal components are lost, especially those in a highlight part and a shadow part. Consequently, the feature of the image is significantly degraded and accuracy in the feature quantity detection deteriorates.